Eddy current testing (ET) is a non-destructive method for inspecting metallic parts. It involves the use of an electromagnetic probe with one or more coils that move across the surface of a test piece, inducing electrical currents (eddy currents) in the test piece near the probe. Since discontinuities in a metal such as a flaw can alter the flow of eddy currents, thereby changing the electromagnetic impedance of the coil, measurement of probe impedance can provide the information needed to locate and identify discontinuities in the material.
The processing of ET signals is often performed by human analysts, who visually inspect signals in search of patterns that indicate the presence of a material defect. In some cases, an indication is visually prominent, thus easily identified, while, in other cases, an indication pattern is either similar to or obscured by signal background patterns, thus difficult to locate. Since signal inspection is a visual skill based on subjective judgement, it is not unusual to find that different analysts inspecting the same signal sometimes come to different conclusions; even the same analyst may produce different conclusions for the same data inspected at different times.
Prior attempts have been made to automate eddy current inspection procedures. In particular, such attempts are disclosed in U.S. Pat. No. 4,628,261, issued to Huschelrath et al.; U.S. Pat. No. 4,821,204, issued to Huschelrath et al.; U.S. Pat. No. 4,979,124, issued to Sachse et al. and U.S. Pat. No. 5,144,565, issued to Brown et al.
A common feature of these methods is their analytic approach, as opposed to the utilization of visual methods. Another common feature of these prior methods is the requirement of pre-test learning from similar physical systems. Consequently, significant amounts of input information in these methods must be derived from external test systems. Prior methods assume that there is a set of measurable geometric and/or electrical parameters whose values suffice to uniquely locate and interpret an indication. The present method makes no such assumption, does not attempt to interpret indications, and claims only detection capability.
It is an object of the present invention to provide automated eddy current scanning, thereby rendering ET signal analysis more objective and repeatable. The invention is based on general methods that emulate two important characteristics of human visual skills: global perception and reasoning with shapes. Global perception means that an entire signal, rather than individual data points, is treated as the fundamental object of processing. Reasoning with shapes means that signals are analyzed by considering their geometric form rather than their quantitative behavior. This approach is appropriate for automated ET scanning because it emulates the methods used by ET analysts: applying visual skills to identify visual patterns.
It is a further object of the present invention to provide processing of ET signals in which human visual characteristics are emulated through the use of two image processing methods, mathematical morphology and pattern recognition. Under such processing, information is extracted, and indications identified, by performing appropriate sequences of morphological processing using geometric filters. Sequential filtering is the practical implementation of the concept of global perception using visual shape-reasoning.